1. Field of the Invention
The present invention relates to a control device for a continuously variable transmission for use in a vehicle.
2. Description of the Related Art
The continuously variable transmission (CVT) for use in a vehicle includes a primary pulley provided at an input shaft and a secondary pulley provided at an output shaft and controls its speed ratio by changing the winding diameter of a driving belt or the like placed around these pulleys. The primary and secondary pulleys each include a fixed sheave and a movable sheave opposing the fixed sheave. The movable sheave is moved in the axial direction, which allows the pulley groove width to be changed, so that the winding diameter of the driving belt can be controlled.
The continuously variable transmission controls the winding diameter of the driving belt by one of the pulleys (for example the primary pulley) and restrains the slipping of the driving belt by the other pulley (for example the secondary pulley). When the speed ratio is controlled by the primary pulley, a target speed ratio is set by referring to a characteristic map based on the throttle opening degree or the vehicle speed, and the pulley groove width of the primary pulley is controlled according to the target speed ratio. When the slipping of the driving belt is controlled by the secondary pulley, a target clamp force is set based on the target speed ratio and an input torque, so that the pulley groove width of the secondary pulley is controlled according to the target clamp force.
The primary and secondary pulleys have their pulley groove widths controlled in this way, primary pressure regulated through a speed ratio control valve is supplied to the hydraulic fluid chamber of the primary pulley, while line pressure regulated through a line pressure control valve is supplied to the hydraulic fluid chamber of the secondary pulley. The speed ratio control valve or the line pressure control valve regulates the primary pressure or the line pressure in response to a control signal from an electronic control unit, and a hydraulic control circuit has a fail-safe function in order to secure the minimum driving performance as well as the driving safety even when the speed ratio control valve or the line pressure control valve becomes uncontrollable in a failed state. According to a proposed control device, the speed ratio is fixed or controlled to the overdrive side in a failed state, so that abrupt down-shifting is prevented and the safety is secured. This is because when the speed ratio control valve attains a failed state in particular, and the hydraulic fluid is abruptly discharged from the primary pulley, the vehicle can be decelerated by abrupt down-shifting (see for example JP-A-2001-12598).
When the speed ratio control valve attains a failed state and the speed ratio is controlled to the overdrive side, it is difficult to re-start the vehicle after stopping the vehicle. Therefore, according to another proposed control device for a continuously variable transmission, when the speed ratio control valve attains a failed stated, the supply of the hydraulic fluid is controlled through a manual valve in association with selecting operation, so that the speed ratio may be controlled to a low state in order to improve the starting performance (see for example JP-A-6-42625). According to yet another proposed control device, when a large load is applied on the driving belt as the line pressure is set to a high value because of a failed state attained by the line pressure control valve, the speed ratio is approximated to 1.0 to alleviate the load on the driving belt (see for example JP-A-2001-330135).
The continuously variable transmissions disclosed by JP-A-2001-12598 JP-A-6-42625, and JP-A-2001-330135 each include a single cylinder type primary pulley including one hydraulic fluid chamber. However, the primary pulley structure is not limited to such a single cylinder type and a double cylinder type having two hydraulic fluid chambers has been developed. The continuously variable transmission including the double cylinder type primary pulley also needs a fail-safe function in order to secure the minimum necessary driving performance as well as the driving safety. However, it has been extremely difficult to apply the conventional fail-safe function to the transmissions.